In silicon integrated circuit (IC) fabrication, it is often necessary to isolate devices that are formed in a single substrate from one another. The individual devices or circuit components subsequently are connected to other circuit elements to create a specific circuit configuration.
A CMOS image sensor circuit includes a focal plane array of pixels, each one of the pixels includes a photo-conversion device, e.g., a photogate, photoconductor, or photodiode having an associated charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel may include a transistor for transferring charge from the charge accumulation region to a diffusion node and a transistor for resetting the diffusion node to a predetermined charge level prior to charge transference. The pixel may also include a source follower transistor for receiving and amplifying charge from the diffusion node and an access transistor for controlling the readout of the pixel contents from the source follower transistor. In some arrangements, the transfer transistor is omitted and the charge accumulation region is coupled with the diffusion node.
In a CMOS image sensor, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the diffusion node accompanied by charge amplification where a transfer transistor is used; (4) resetting the diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a reset signal and a signal representing pixel charge from the diffusion node. The charge at the floating diffusion node is typically converted to a pixel output voltage by the source follower output transistor.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol.41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524 which describe operation of conventional CMOS image sensors, the contents of which are incorporated herein by reference.
Reducing dark current in a photodiode is important in CMOS image sensor fabrication which use photodiodes as the photoconversion device. One source of dark current is leakage in the charge collection region of a photodiode, which is strongly dependent on the doping implantation conditions. Other sources of dark current include unwanted electrons from peripheral circuits and electron generation from infrared photons. Dark current is also caused by current generated from trap sites inside or near the photodiode depletion region; band-to-band tunneling induced carrier generation as a result of high fields in the depletion region; junction leakage coming from the lateral sidewall of the photodiode; and leakage from isolation corners, for example, stress induced and trap assisted tunneling.
Another problem often associated with photodiodes is that of blooming. That is, under illumination, electrons can fill up an n-type well. Under saturation light conditions, the n-type well can completely fill with electrons, and the electrons will then bloom to adjacent pixels. Blooming is undesirable because it can lead to, for example, the presence of a bright spot on the image.